A causal diamond in a Lorentzian space-time is the intersection of the backward lightcone of a point and the forward lightcone of a point in its causal past. The geodesic between the points is the timelike trajectory of maximal proper time, and the holographic screen of the diamond is the maximal area \(d - 2\)  surface in the null foliation of the diamond's boundary. The covariant entropy principle (CEP) is a generalization[1]} of the Bekenstein-Hawking-Gibbons formulas[2]} for the entropies of black holes and of de Sitter(dS) space to arbitrary diamonds in arbitrary space-times. It was anticipated in the work of 't Hooft[3]} and in Jacobson's 1995 derivation[4]} of Einstein's field equations from the first law of thermodynamics.
